Preliminary interpretations of geologic results obtained from boreholes UE25a-4,-5,-6, and -7, Yucca Mountain, Nevada Test Site

Abstract

A study of the subsurface geology was undertaken in association with the Nevada Nuclear Waste Storage Investigations to identify any near-surface structural features that may be present beneath one of four linear northwest-trending washes that transect the northeastern part of Yucca Mountain at the Nevada Test Site. Four drill holes were continuously cored to depths of about 500 feet. In descending order, the drill holes penetrated the Tiva Canyon, Yucca Mountain, Pah Canyon, and Topopah Spring Members of the Paintbrush Tuff of Tertiary age. These units consist almost entirely of nonwelded to densely welded rhyolitic ash-flow tuffs, separated by thin beds of air-fall and reworked tuff.The Yucca Mountain and Pah Canyon Members are of particular interest because their total thicknesses were penetrated in each of the four holes. A comparison of thickness variations of both units indicate progressive thinning from northwest to southeast, which appears consistent with that of typical thinning away from source areas located to the northwest.An analysis of oriented core samples taken within the densely welded Topopah Spring Member in each drill hole indicates two complementary sets of high-angle fractures striking N. 12° W. and N. 37° E. Compilations of fracture frequencies show that core from drill holes located close to the wash margins is more fractured than core from drill holes near the center of the wash. The northeasterly margin of the wash shows evidence of minor faulting inferred from the presence of a zone of en echelon shear fractures found in core from hole UE25a-4. These shear fractures are high angle, show evidence of brecciation, and most strike parallel to the wash lineation. Identification of structure along the southwest edge of the wash was independently obtained through the analysis of eutaxitic structure within core of the Topopah Spring Member, and confirmed by paleomagnetic studies within the same member. Measurements of foliation attitudes in drill holes UE25a-4, -5, and -7 indicate a mean strike direction of about N. 10° W. within the wash. In marked contrast, the mean foliation attitude in UE25a-6, located outside the wash trend, strikes N. 23° E. This 33° change in strike occurs abruptly over a distance of about 1,000 feet. The difference between these orientations, as well as the absence of vertical offset along shear fractures in UE25a-4 and between drill holes seem best explained by tectonic rotation that occurred about a near-vertical axis as a result of minor strike-slip or oblique-slip movement within the wash. As no abrupt changes in thickness were apparent in the Yucca Mountain and Pah Canyon Members, the magnitude of displacement along the structure is considered to be of small scale. Additional subsurface information, particularly in the area underlying ridges northeast of the wash, is needed not only to confirm the existence of the structure but also to aid in determining the amount of lateral movement.

Status of understanding of the saturated-zone ground-water flow system at Yucca Mountain, Nevada, as of 1995

“…Infrequently, Fortymile Wash has carried water to western Jackass Flats and even less frequently into the Amargosa Desert (Squires and Young, 1984). Water flowing down Fortymile Wash quickly infiltrates into the channel deposits; therefore, major flows need to occur for water to reach the lower sections.…”

“…Simulations could be made for floods with recurrence intervals of perhaps 500 or 1,000 years (or longer). Squires and Young (1984) estimated various characteristics of peak flow for 100-year, 500-year, and maximum regional floods for several sections of Fortymile Wash. However, estimating the volumes of such rare floods, as well as the recharge associated with them would be difficult.…”

Bedrock geologic map of the Yucca Mountain area, Nye County, Nevada

“…The main trace of the fault is intersected in the subsurface in borehole a # 4 {Spengler and Rosenbaum, 1980) and in the ESF (Steven C. Beason, U.S. Bureau of Reclamation, oral commun., 1995). The fault is exposed at the surface on Tonsil Ridge northwest of borehole H-1.…”

Preliminary hydrogeologic assessment of boreholes UE-25c #1, UE-25c #2, and UE-25c #3, Yucca Mountain, Nye County, Nevada

“…Because most of the fractures at Yucca Mountain are steeply dipping to vertical, it is possible that in the vertical C-holes, specific fracture sets and, consequently, fracture spacing in general could be underrepresented. Comparisons with fracture data obtained by pavement mapping (Barton and Larsen, 1985;Barton and others, 1989), transects (U.S. Geological Survey, 1984), angled and horizontal coring (Spengler and Rosenbaum, 1980;Norris and others, 1986), and vertical coring (Scott and Castellanos, 1984) were made to substantiate analyses of fracture data from the C-holes or indicate the margin of error in these analyses.…”